Plastic tanks are widely used for liquid storage because they are economical, lightweight, durable and resistant to corrosion & chemicals. In agriculture, industry and residential applications, plastic tanks are often used for the storage of water for household use, animal consumption and use in agricultural operations such as chemical crop spraying.
In the manufacture of molded plastic tanks for liquid storage, the use of unpigmented or light colored resins is often favored. Costs and process complexities associated with the use of pigments can be reduced or eliminated. A further significant advantage of tanks with a light colored exterior is that they do not absorb heat from sunlight to the extent that a dark colored tank does, but rather reflect a substantial portion off the outer surface. Thus a light colored tank is less susceptible to sunlight-induced heating and softening of the plastic. A light colored tank stays cooler in the sun and as a result the wall thickness can be reduced compared to a dark colored tank while retaining its structure.
Under sunlight-induced heating, a dark colored tank may seriously deform or collapse under its own weight when empty, or under the stress of retaining means when tied down for transport. Prevention of such deformation or collapse is conventionally accomplished through the use of structural stiffening provided by increased wall thickness or reinforcing ribs. However, the use of additional material renders a tank heavier and more costly, and reinforcing ribs require more costly and complex molds, which also increases the cost of the tank.
A significant drawback of a light colored plastic tank used for water storage is that the tank walls are translucent. Sunlight can pass through the wall of the tank and promote the growth of algae in water stored in the tank. Such tanks are not always completely drained between uses, and algae growth in a small amount of water retained in the tank over a significant period of time can be considerable.
This algae growth is highly undesirable. Such tanks are commonly used to haul and store water for human and animal consumption, water for household use, and water for agricultural chemical spraying. Such applications require relatively pure and uncontaminated water. Not only can algae be toxic to animals and humans, but it plugs spraying equipment and can interfere with the action of agricultural chemicals.
Plastic water tanks are commonly manufactured by rotational molding processes. It is known in the rotational molding art to provide an article with an inner layer of one plastic material and an outer layer of a different plastic material, as is disclosed in U.S. Pat. No. 4,548,779 to Steinberg et al.